At present, RNA interference (RNAi) technique is frequently utilized for life science researches, and usefulness thereof has been confirmed widely. RNAi refers to a phenomenon of suppression of gene expression by degradation of mRNA, which is specific to a double-stranded RNA sequence. Ever since the report in 2001 that a low molecular weight double-stranded RNA having 21 bases can mediate RNAi in a mammalian cell (non-patent document 1), siRNA (small interference RNA) has been frequently used as a method for suppressing expression of a target gene. siRNA is expected to be applicable to pharmaceutical products and treatment of various intractable diseases including cancer.
miRNA (microRNA) is an endogenous, non-coding RNA of about 20-25 bases encoded on the genome. miRNA is transcribed from an miRNA gene on the genomic DNA first as a primary transcript (Primary miRNA, hereinafter to be referred to as “Pri-miRNA”) having a length of about several hundred to several thousand bases, and then processed into a pre-miRNA (precursor miRNA) having a hairpin structure of about 60-70 bases. Thereafter, it moves from the nucleus into the cytoplasm and is further processed into a double-stranded mature miRNA of about 20-25 bases. It is known that one strand of the double-stranded mature miRNA forms a complex with a protein called RISC and acts on mRNA of a target gene to inhibit translation of the target gene (see, for example, non-patent document 1).
There are 1000 or more kinds of miRNAs known for human, mouse and the like, each of which is suggested to regulate expression of plural target genes, and be involved in various biological phenomena such as growth, differentiation and the like of the cell. For example, miRNA involved in differentiation of hematopoietic cell and nerve cell, and the like has been reported (see, for example, non-patent document 2). In addition, there are reports on miRNA involved in cancer cell growth, where utilization of miRNA expression pattern for clinical cancer diagnosis, and a treatment method of cancer which suppresses cancer cell growth by suppressing expression of miRNA have been proposed (see, for example, patent documents 1 and 2).
On the other hand, it is widely known that the highest hurdle for the cancer chemotherapy is drug resistance acquired by cancer cells. As the mechanism of acquiring resistance, various theories have been proposed including increased expression of ABC transporter which is one kind of drug transporter, increased expression of RPN2 gene which is a glycosylation enzyme (non-patent document 3) and the like, and a strong ability of cancer stem cells to excrete anti-cancer agents is drawing attention in recent years (non-patent document 4).
Expression of microRNA-27b (miR27b) is observed in various cancer cells such as breast cancer cells (non-patent document 5), prostate cancer cells (patent document 3), pancreatic cancer cells (patent document 4) and the like. As for physiological function, non-patent document 5 states that forced expression of miR27b in ZR75 cell, which is a breast cancer low-metastatic cell, promotes cell proliferation. However, there is no document that has reported the relationship between miR27b and drug-resistant cancer cells.